The present invention relates to a method for transmitting data asymmetrically in a radiocommunications system which has at least one fixed unit whose coverage area is divided into sectors, and within each sector a group of remote units is located.
The fixed unit communicates with the remote units making joint use of time division multiple access (TDMA) and time division duplex (TDD) techniques. The data transmission method is of special, but not exclusive application, in a digital cordless telecommunications system (DECT) for the avoidance of interference between fixed units located at the same site and which cover boundary sectors of a given coverage area.
A radiocommunications system has its coverage area divided into several sectors such that each sector is covered by a radio receiver and a radio transmitter, both being incorporated in a fixed unit, so that several fixed units are located at the same site, each of them covering a different sector and these been adjacent.
A set of remote units is located in each sector so that each fixed unit communicates with the remote units situated within the sector associated with it. To implement communication joint use is made of time division multiple access (TDMA) and time division duplex (TDD) techniques.
The same frequency band is employed in all sectors. The carrier frequency is divided into time slots, which are grouped into frames, so that the allocation of a time slot in one direction of the communication, for example, the uplink direction, implies the automatic allocation of a duplex time slot, in the other direction of communication, in this case the downlink direction.
Consequently, the same radio channel (pair formed by a time slot and a frequency) can be used simultaneously for two communications in different sectors, adjacent or not.
The radio channel employed in a communication is selected by the remote unit, after analysis and evaluation of the power present in each downlink slot, and at each of the frequencies, making use of the dynamic channel allocation (DCA) procedure.
Though this is the most frequent case, it is possible to set up at least one asymmetric communication. In this case, at least two time slots of the same radio channel, downlink time slot and duplex slot, are used simultaneously for the downlink direction of the transmission.
When this type of communication is set up, interference arises in sectors adjacent to the sector in which the asymmetric communication has been set up, rendering inaudible a signal received over the same uplink channel used in a sector adjacent to the sector in which the asymmetric communication has been established.
This occurrence is due to the fact that the radio transmitter in the sector in which the asymmetric communication is set up uses a higher power level to transmit in the downlink direction than the power with which the radio receiver of the adjacent sector receives the bursts corresponding to the communication in process and transmitted from the radio transmitter of the remote units implicated in the communications.
This power difference is so great that, even though no remote unit in the sector or adjacent sectors select the same time slot on the same frequency for its symmetrical communications, the proximity with which the hexagonally arranged antennas are mounted for sweeping the different sectors, and the not entirely exact response of the filters, result in interference arising in the same time slot (used symmetrically) on another frequency, or in the adjacent time slots.
Consequently, there is a need to develop a method for transmitting information inside a time slot allocated to a communication, so that the possibility of experiencing interference due to its duplex time slot being used in an asymmetric communication in an adjacent sector is reduced, obtaining correct reception of the information contained in the time slot in a radio receiver of the fixed unit corresponding to another sector.
To overcome the problems described above, a method of asymmetric data transmission is proposed for use by a fixed unit whose coverage area is divided, at least, into a first sector and a second sector.
In each sector a set of fixed and/or mobile remote units is located, which communicate by radio with a first fixed unit and a second fixed unit, respectively, so that both fixed units are located one beside the other and use the same frequency band.
When one of the two fixed units is transmitting asymmetric data such as IP (Internet Protocol) packets to a remote unit located in its associated sector, i.e., it occupies the pair of time slots corresponding to a radio channel, that is the time slot corresponding to the downlink half of the frame and the duplex time slot corresponding to the uplink half of the frame; interference is produced in the fixed unit whose sector is adjacent to the sector in which the asymmetric communication is taking place.
The method of the invention avoids interference, since controller means perform a checking process on all communication links engaged with calls going on, as well as on all requests to set up new calls in each of the sectors into which the coverage area is divided, in order to determine whether there are asymmetric communications and on which channels they have been set up.
Once the radio channels have been identified, the controller means insert a message with the identity of the channels occupied with asymmetric communications in the broadcast channel that each fixed unit has set up in order to prevent the remote units from initiating a communication over said channels.
Thus, a remote unit is prevented from selecting as best, a time slot that can be in use for an asymmetric communication in an adjacent sector. The controller means update the broadcast information in terms of the variation in traffic of the radiocommunications system.